1. Field of the Invention
The present invention relates to a novel process for the preparation of aqueous solutions of diazonium salts of the aromatic amines.
2. Description of the Prior Art
It is conventional to prepare diazonium salts in aqueous solutions by gradually introducing an alkali metal nitrite into a medium containing the amine salt and a free acid. Such processes are satisfactory as regards the yields of the diazonium salt; however, these processes exhibit a significant disadvantage from an industrial point of view. In fact, during the reaction, inorganic species (typically sodium chloride and the like) are produced in stoichiometric amounts and same must be separated therefrom and treated for the purpose of reducing pollution.
A few diazotization techniques employing nitrogenous vapors as the diazotizing agent have been described in the literature. Diazotization by nitrogenous vapors was essentially carried out in an organic medium (ethyl ether, benzene or dimethylforamide) on the free amine or on the amine salt (hydrochloride or the like). It was demonstrated that nitric oxide, NO, was by far the best diazotizing agent [J. Rigaudy et al, Comptes-Rendus de l'Academie des Sciences, 258 (4) 1,529-31 (1964), 258 (19) 4,799-802 (1964) and 261 (25) 5,516-19 (1965)]. It was also demonstrated in these same publications that poor results were obtained when using nitrogen peroxide, NO.sub.2, or nitrogen trioxide, N.sub.2 O.sub.3, as the diazotizing agent.
Japanese Patent No. 4,331(67) of Sankyo Chemical Industries, bearing date of May 11, 1964, describes a process for the preparation of diazonium hydrochloride in aqueous solution by the introduction of pure NO into a solution of aniline hydrochloride containing free hydrochloric acid. It is particularly recommended in this patent to avoid contamination of the nitric oxide, NO, with nitrogen peroxide, NO.sub.2.
Furthermore, the note of A. Sanfourche and J. Bureau, Comptes-Rendus de l'Academie de Sciences, 202, 66-69, 6/1 (1936), reflects that it is possible to employ nitrogen peroxide, NO.sub.2, or an equimolecular mixture of NO and NO.sub.2 as the diazotizing agent.
When the diazotization processes are carried out by reacting NO or NO.sub.2, or an equimolecular mixture of NO and NO.sub.2, with an aqueous solution of amine salt, optionally containing some free acid, it has been found that the yields of diazonium salt are on the order of 50%, relative to the diazotizing agent employed. Moreover, it too has been observed that nitric oxide, NO, is a slow diazotizing agent from the point of view of reaction kinetics. Furthermore, the use of nitrogen peroxide, NO.sub.2, entails the formation of nitric acid, and this if of course a disadvantage from an industrial point of view because of the production and steady increase in the amount of such additional strong acid. Thus, there exists a need in this art for an alternative diazotization reaction, devoid of the aforenoted drawbacks.